The present invention relates generally to capacity control for refrigeration systems, and more particularly to a process and system for varying the capacity of a refrigeration system employing an economizer.
In refrigeration systems, a refrigerant gas is compressed by a compressor and passed to a condenser where it exchanges heat with another fluid such as the ambient air. From the condenser, the pressurized refrigerant passes through an expansion device and then to an evaporator, where it exchanges heat with another fluid that is used to cool an environment. The refrigerant returns to the compressor from the evaporator and the cycle is repeated.
Economizer circuits are utilized in refrigeration systems to provide increased cooling capacity, and also to increase efficiency and performance of the system. An economizer circuit is sometimes incorporated just downstream of the condenser, where it produces a cooling effect on the pressurized liquid refrigerant flowing from the condenser on its way to the expansion device and the evaporator. By lowering the pressure of some liquid refrigerant sourced from the condenser and then returning the lower pressure refrigerant to the main liquid refrigerant line upstream of the primary expansion device, the economizer lowers the enthalpy of the liquid refrigerant, thereby increasing the differential enthalpy achieved by the system.
Economizer circuits typically include a refrigerant line communicably connected to the condenser or to the main refrigerant line downstream of the condenser, an economizer expansion device, and an economizer heat exchanger. A flash tank can easily serve as a heat exchanger in an economizer circuit. In flash tank economizer circuits, the economizer expansion device is provided upstream of the flash tank, and is communicably connected to an inlet provided in the upper portion of the flash tank. Liquid refrigerant flows through the expansion device, through the inlet, and into the flash tank. Upon passing through the expansion device, the liquid refrigerant experiences a substantial pressure drop, whereupon, at least a portion of the refrigerant rapidly expands or “flashes” and is converted from a liquid phase to a gas phase. The unflashed liquid refrigerant gathers at the bottom of the tank for return to the main refrigerant line upstream of the primary expansion device. Gas phase refrigerant is returned to the compressor, whether to compressor suction or to an intermediate stage of compression. As a result of the intermediate pressure of refrigerant gas in the flash tank, the gas returned to the compressor requires less compression, thereby increasing compressor efficiency.
To further control the cooling or heating capacity of the system, it is desirable to have the capability of turning the economizer circuit on or off or activating or deactivating the economizer circuit. Thus, a shut-off valve can be provided in some known economizer circuits, as further described below.
To create variable capacity control and to maintain a tight tolerance in suction pressure and/or refrigerated space temperature, it is typical to control capacity by cycling or unloading compressors. Unloading of screw compressors typically involves providing at least one capacity control valve at a predetermined stage of compression. Opening the capacity control valve allows a portion of the refrigerant gas to escape from the compression chamber, leaving less gas for compression. Thus, the load on the screw compressor is decreased, thereby increasing compressor efficiency.
There exist several known capacity control valves for reducing system capacity or “unloading” of compressors. For example, slide valves and plug valves can be used to open and close a capacity control opening that connects the compression chamber to a bypass circuit that returns gas from an intermediate stage of the screw compressor to the suction inlet, or to a lower-pressure stage of the screw compressor. The bypass circuit and capacity plug valves provide a single predetermined or “stepped” capacity decrease. This is because plug valves operate in just two positions—fully open, and fully closed. When open, the capacity plug valve channels some gas from its fixed load point in the compressor through the bypass channel back to compressor suction. When closed, the capacity plug valve allows the compressor to operate at full compression capacity. Because capacity plug valves can only operate in two positions, opening the valve provides fixed unloading of capacity, but does not provide for any variable unloading of capacity.
In contrast, slide valves provide for variable control of a capacity control opening in a compression chamber. Slide valves generally include a flat slide plate that is exteriorly slideably mounted over a capacity control opening. Slide valves can be hydraulically controlled to adjustably cover the capacity control opening, thus adjustably unloading to reduce system capacity. One drawback to slide valves is that the inherent structural limitations make it difficult, if not impossible, to eliminate compressor leakage around the slide valve even when fully closed. Such slide valve leakage can seriously hamper system efficiency, and can also limit the peak capacity of the system. In addition, slide valves can be difficult and expensive to machine.
There are several known systems employing both an economizer circuit and a capacity control valve for unloading the compressor. For example, U.S. Pat. No. 5,816,055 to Öhman is directed to apparatus and methods for controlling the efficiency and capacity of an economizer circuit having a flash tank heat exchanger. Öhman discloses the use of an adjustable control valve in an economizer circuit that regulates the flow of gaseous refrigerant from the flash tank to the compressor. The control valve also simultaneously controls a bypass return channel from the compressor to suction. Öhman discloses that system capacity can be maximized by opening the valve so as to allow higher gas return from the economizer flash tank to the compressor, which opening simultaneously fully closes the bypass return channel. Modulating the adjustable valve to decrease gas flow from the economizer, thus opening the bypass channel, decreases system capacity to between 75% and 100%. Finally, fully closing the adjustable valve shuts off the economizer circuit and leaves the bypass channel fully open to minimize system capacity to between 40% and 75%. Further lowering of capacity to 25% is also disclosed by shaping of the valve body and the bypass channel.
By way of further example, U.S. Pat. No. 6,385,980 to Sienel is directed to apparatus and methods for controlling the efficiency and capacity of a flash tank in an economizer circuit. The Sienel patent discloses the use of expansion valves to control the flow of refrigerant into and out of the flash tank, thereby regulating the amount of refrigerant stored in the flash tank, and in turn controlling the amount of refrigerant in the condenser and the high pressure side of the system. A first expansion valve regulates the flow of liquid refrigerant from the condenser into the flash tank, and a second expansion valve regulates the flow of liquid refrigerant charge out of the flash tank. The Sienel patent further discloses that an additional control valve can be provided to control the flow of refrigerant gas from the flash tank to the compressor, and that closing that particular valve will turn off the economizer by blocking vapor refrigerant from exiting the flash tank and entering the compressor.
Lastly, U.S. Pat. No. 6,385,981 to Vaisman is directed to a method of reducing cooling capacity in a refrigeration system having a main circuit, an economizing circuit, and a capacity control bypass circuit. The main circuit comprises a compressor, a condenser unit, an expansion device, an evaporator unit, connecting piping and appropriate refrigeration control. The compressor includes an economizer port located in the compression region, and a variable flow valve associated with the economizer port. The economizer circuit includes a first solenoid valve, an additional expansion device and an economizing heat exchanger. The bypass circuit also has a solenoid valve that acts as a shut-off for the bypass circuit. A control system activates the valves based on a capacity demand. The system disclosed in Vaisman includes a single compressor port that controls access to both the bypass circuit and to the economizer circuit, to thereby prevent the economizer and the capacity control bypass circuit from being operated simultaneously.
Therefore, there exists a continuing need for an economizer-equipped refrigeration system that provides for operation of at least one capacity control valve controlling an independent bypass circuit simultaneously with the operation of an independent modulating control valve to variably control a separate economizer circuit to permit efficient, flexible, reliable, and variable system capacity control, without leakage that can reduce system peak capacity.